1,721,513 research outputs found
Accretion in the ρ-Ophiuchi pre-main sequence stars
No full textAims.The aim of this paper is to provide a measurement of the mass accretion rate in a large, complete sample of objects in the core of the star forming region ρ Oph.
Methods: .The sample includes most of the objects (104 out of 111) with evidence of a circumstellar disk from mid-infrared photometry; it covers a stellar mass range from about 0.03 to 3 M_⊙ and it is complete to a limiting mass of 0.05 M_⊙. We used J and K-band spectra to derive the mass accretion rate of each object from the intensity of the hydrogen recombination lines, Paβ or Brγ. For comparison, we also obtained similar spectra of 35 diskless objects.
Results: .The results show that emission in these lines is only seen in stars with disks, and can be used as an indicator of accretion. However, the converse does not hold, as about 50% of our disk objects do not have detectable line emission. The measured accretion rates show a strong correlation with the mass of the central object (dot M_acc ∝ M_star1.8±0.2) and a large spread, of two orders of magnitude at least, for any interval of M_star. A comparison with existing data for Taurus shows that the objects in the two regions have similar behaviour, at least for objects more massive than ∼ 0.1 M_⊙. The implications of these results are briefly discussed
Chemical composition of the young open clusters IC 2602 and IC 2391
No full textContext. Galactic open clusters have been long recognized as one of the best tools to investigate the chemical content of Galactic disk and its time evolution. In the last decade, many efforts have been directed to chemically characterize the old and intermediate age population; surprisingly, the chemical content of the younger and close counterpart remains largely undetermined.
Aims. In this paper we present the abundance analysis of a sample of 15 G/K members of the young pre-main sequence clusters IC 2602 and IC 2391. Along with IC 4665, these are
the first pre-main sequence clusters for which a detailed abundance determination has been carried out so far.
Methods. We analyzed high-resolution, high spectra acquired with different instruments (UVES and CASPEC at ESO, and the echelle spectrograph at CTIO), using MOOG and equivalent width measurements. Along with metallicity ([Fe/H]), we measured Na
U-band study of the accretion properties in the σ Orionis star-forming region
No full textThis paper presents the results of an U band survey with FORS1/VLT of a large area in the σ Ori star-forming region. We combine the U-band photometry with literature data to compute accretion luminosity and mass accretion rates from the U-band excess emission for all objects (187) detected by Spitzer in the FORS1 field and classified by Hernandez et al. as likely members of the cluster. The sample
̇
stars range in mass from ∼0.06 to ∼1.2 M⊙; 72 of them show evidence of disks and we measure mass accretion rates Macc between
<10−11 and few 10−9 M⊙/y, using the colors of the diskless stars as photospheric templates. Our results confirm the dependence of ̇
of Macc for any value of the stellar mass is ∼2 orders of magnitude. We discuss the implications of these results in the context of ̇
Macc on the mass of the central object, which is stronger for low-mass stars and flattens out for masses larger than ∼0.3 M⊙; the spread ̇
disk evolution models. Finally, we analyze the relation between Macc and the excess emission in the Spitzer bands, and find that at ̇
Macc ∼ 10−10 M⊙/y the inner disks change from optically thin to optically thick
Accretion properties of T Tauri stars in σ Orionis
No full textAccretion disks around young stars evolve in time with time scales of a few million years. We present here a study of the accretion properties of a sample of 35 stars in the ∼ 3 million-year-old star-forming region σ Ori. Of these, 31 are objects with evidence of disks, based on their IR excess emission. We use near-IR hydrogen recombination lines (Paγ) to measure their mass accretion rate. We find that the accretion rates are significant lower in σ Ori than in younger regions, such as ρ Oph, consistently with viscous disk evolution. The He I 1.083 μ m line is detected (either in absorption or in emission) in 72% of the stars with disks, also providing evidence of accretion-powered activity in very low accretors, where other accretion indicators disappear
Accretion in ρ Ophiuchus brown dwarfs: infrared hydrogen line ratios
No full textContext: .Mass accretion rate determinations are fundamental for an understanding of the evolution of pre-main sequence star circumstellar disks.
Aims: .Magnetospheric accretion models are used to derive values of the mass accretion rates in objects of very different properties, from brown dwarfs to intermediate-mass stars; we test the validity of these models in the brown dwarf regime, where the stellar mass and luminosity, as well as the mass accretion rate, are much lower than in T Tauri stars.
Methods: .We have measured nearly simultaneously two infrared hydrogen lines, Paβ and Brγ, in a sample of 16 objects in the star-forming region ρ-Oph. The sample includes 7 very low mass objects and brown dwarfs and 9 T Tauri stars.
Results: .Brown dwarfs where both lines are detected have a ratio Paβ/Brγof ~2. Larger values, ⪆3.5, are only found among the T Tauri stars. The low line ratios in brown dwarfs indicate that the lines cannot originate in the column of gas accreting from the disk onto the star along the magnetic field lines, and we suggest that they form instead in the shocked photosphere, heated to temperatures of ~3500 K. If so, in analogy to veiling estimates in T Tauri stars, the hydrogen infrared line fluxes may provide a reliable measure of the accretion rate in brown dwarfs
Chemical composition of the Taurus-Auriga association
No full textContext. The Taurus-Auriga association is perhaps the most famous prototype of a low-mass star forming region, surveyed at almost all wavelengths. Unfortunately, like several other young clusters/associations, this T association lacks an extensive abundance analysis determination.
Aims. We present a high-resolution spectroscopic study of seven low-mass members of Taurus-Auriga, including both weak-lined and classical T Tauri stars designed to help robustly determine their metallicity.
Methods. After correcting for spectral veiling, we performed equivalent width and spectral synthesis analyses using the GAIA set of model atmospheres and the 2002 version of the code MOOG.
Results. We find a solar metallicity, obtaining a mean value of [Fe/H] = −0.01 ± 0.05. The α-element Si and the Fe-peak one Ni confirm a solar composition. Our work shows that the dispersion among members is well within the observational errors at variance with previous claims. As in other star forming regions, no metal-rich members are found, reinforcing the idea that old planet-host stars form in the inner part of the Galactic disc and subsequently migrate
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
- …
